Combine the onions, shallots and green onions in a mixing bowl with the vinegar, oil, sugar, 1/2 teaspoon salt, 1/4 teaspoon black pepper. Mix well and set aside at room temperature.

In a small bowl, whisk together the mayonnaise, Creole seasoning, and lemon juice and set aside.

In a shallow mixing bowl, whisk together the cornstarch with 1 1/2 teaspoons salt, and the egg white. Add the water and oil and whisk well. Agitate the mixture just before you add the crabs.

Rinse the crabs in cold water. Use kitchen shears to remove the eyes and mouth of the crab. Flip the crab over and remove the apron. Lift the top of the shell and remove the lungs. Lay the crabs on a paper towel to drain.

Preheat a deep fryer to 350º F.

Dip the crabs into the batter and then fry the crabs in batches about 3 minutes per side. Transfer the crabs to a paper towel lined plate.

Spread the mayonnaise on both sides of the poboy bread. Place one crab on the bottom half of the poboy, top with 1/4 cup of the onion slaw and then place the top on the sandwich.

Any leftover onion slaw can be stored in a resealable container in the refrigerator for 2 to 3 days.